Fibers derived from 5(6)-amino-2-(p-aminophenyl)benzimidazole (DAPBI), para-phenylenediamine (PPD) and terephthaloyl dichloride (TCl or T, also commonly referred to as terephthaloyl chloride) are known in the art. Such copolymers are the basis for a high strength fibers manufactured in Russia, for example, under the trade names Armos® and Rusar®. See, Russian Patent Application No. 2,045,586.
One disadvantage to the use of copolymers having DAPBI as comonomer is that DAPBI is an expensive monomer. One major contributor to the high cost is the formation and purification of the free-base from the intermediate salts, such as sulfuric acid or hydrochloric acid, depending in the chemistry of the synthesis used.